The present invention relates to printed circuit boards of the type having a solder mask over non-reflowable metal and, more particularly, to a method of manufacture of such circuit boards and to the printed circuit boards resulting therefrom.
As is well-known in the art, the manufacture of double-sided printed circuit boards requires the provision of conductive through-holes for interconnecting components on opposite sides of the board or, in the case of multilayer printed circuit boards, for interconnecting the inner layers. The non-conductive surfaces exposed when through-holes are drilled in a non-conductive substrate having metal cladding on both sides must, therefore, be provided with a conductive coating, and this generally is accomplished by a first electroless deposition of copper onto the suitably conditioned through-hole surfaces, followed by electroplating of copper to build up additional thickness.
In application of the actual circuit patterns to the metal-clad board surfaces, it is necessary to employ plating resists so as to prevent all but particular areas of the board (through-holes and/or traces and/or pads and/or other areas) from receiving applied metal platings such as the copper electroplate used in through-hole plating and/or the commonly employed tin-lead alloy etch resist which is applied prior to the step of etching away copper so as to form the appropriate conductive circuit pattern.
The use of tin-lead alloys as etch resists has the disdvantage that resists have to be stripped after the etching process is completed. Such stripping contributes additional costs due to consumption of both time and additional materials as well as presenting environmental problems in disposal of the materials used.
A further problem encountered with the use of tin-lead etch resist is the formation of minute slivers of the metal during the removal process. These slivers can result in short circuits but are virtually impossible to detect even under magnification.
The tin-lead alloy solder which is applied to through-holes and pads in a later step of the fabrication tends to melt under the solder mask which has been applied to the copper circuit pattern to protect the latter. The tin in the alloy tends to migrate from the solder to the underlying copper circuit traces. This can give rise to galvanic action with subsequent corrosion and deleterious effects on the performance of the board. Further, the migration of tin from the solder into the underlying copper leaves the solder rich in lead and thereby structurally weakened and prone to fracture. When the solder forms the point of attachment of a surface mount device (SMD) the possibility of fracture is increased because of the stress placed on the joint due to the difference in thermal coefficient of expansion of the printed circuit board and the SMD and the effect of repeated heating and cooling during the working life of the board.
The various drawbacks recited above are well recognized in the art and attempts have been made to overcome the same. Illustratively, R. C. Clark, SMOBC: Manufacturing Techniques, Circuits Manufacturing, August, 1982 pp. 45-48 and R. H. Clark, Handbook of Printed Circuit Manufacturing, pp. 564-570, Van Nostrand Reinhold Company, New York, 1985, both describe the use of tin alone, tin-nickel alloys, nickel alone and black oxide as etch resists. However, while these alternatives avoid the use of tin-lead alloy resists they are not entirely free from disadvantages. Thus, adhesion of the solder mask to the surface of these alternate metals can be poor unless surface processing is carried out prior to application of the solder mask. This extra step can have a detrimental effect on circuit integrity. Further, the alternative metal resists must be well activated prior to applying the solder mask in order to avoid poor electrical conductivity and/or peeling of the solder mask. In the case of nickel the possibility of minute sliver formation exists with the deleterious consequences discussed above in the case of tin-lead.
J. D. Fellman PC FAB, December 1981, p. 16 and 51-55 describes the use of electroless tin plated etch resists wich, however, are stripped before applying solder mask.
Mack U.S. Pat. No. 4,104,111 describes the use of tin-nickel as an etch resist over the copper circuit traces. However, a cleaning and chemical reactivation of the tin-nickel layer is necessary prior to application of solder mask thereto. Spiers U.S. Pat. No. 3,297,442 employs a layer of gold as an oxidation resistant coating for copper circuit traces but an additional etching resist is employed prior to the etching step. Gottfried U.S. Pat. No. 3,483,615 and Reimann U.S. Pat. No. 4,312,897 also teach the use of gold as an etch resist for printed circuit boards. The use of such an expensive etch resist is obviously to be avoided, if possible.
Ritt et al U.S. Pat. No. 2,959,525 describes the plating of copper circuit patterns with nickel and, optionally, with rhodium but not until after the etching steps in formation of the circuitry have been completed, i.e. the nickel and rhodium layers are not employed as etch resists.
Ohta et al U.S. Pat. No. 4,512,829 describes a process for producing printed circuit boards in which a key step is electroless plating of the hole-defining inner surfaces without deposition of nickel on the copper clad surfaces. Deposition on the latter is avoided because the etch resistance of the nickel would interfere with subsequent etching of the copper.
O'Hara U.S. Pat. No. 4,444,619 employs a palladium/nickel alloy as an etching resist in fabrication of printed circuit boards. Preferred is an alloy containing 65 to 95 percent palladium.
Rendulic et al U.S. Pat. No. 4,436,806 describes the use of liquid polymer photoresists in the fabrication of printed circuit boards. A metallic plate resist, which can be tin, lead, nickel or a combination thereof, is optionally employed to protect the copper circuit pattern during the etching step.
It has now been found that the problems discussed in regard to the use of tin-lead etch resists and in regard to melting of solder under the solder mask can be overcome readily by utilizing metallic lead as the etch resist and thereafter applying solder mask directly over the layer of metallic lead since the lead does not melt at normal soldering temperatures. Further, if a layer of lead is present on the surface of the copper at the various loci to which solder is applied the problems of weakening of soldered joints discussed above are found to be obviated or greatly reduced. It is believed that lead acts as a barrier metal for tin migration from the tin-lead solder to the copper. In addition to overcoming the problems discussed above, the use of lead as the etch resist following by applying solder mask directly over the lead has the advantage of eliminating the need to strip the etch resist.